The present invention relates to a burnishing drill providing both drilling and reaming functions.
There are known various types of burnishing drills which have drilling edges and reaming edges and are adapted to form a hole in a workpiece made of a material such as cast iron. The workpiece is first drilled by the drilling edges which are made of hard metal. The drilled hole is further worked by the reaming edges which have an external diameter that is slightly larger than that of the drilling edges, to provide a relatively smooth inner wall for the finished hole. The reaming edges are located at a shoulder portion of the drill, extend radially outward from the foremost end of the drill and incline axially in the rearward direction. The inclined angle of the reaming edges is generally set within a range of between 35.degree. and 45.degree..
When forming holes of the same size, such burnishing drills often cause dispersion of diameter, surface roughness and circularity of the finished holes. Especially when the feed speed of the drill is relatively high, these burnishing drills tend to provide an inaccurately finished side wall for the drilled hole, because the burnishing drills do not have good centering properties.